A known optically readable disc-shaped information carrier is the CD. The elementary marks of the information layer of the CD comprise tiny pits which are provided along tracks in the information layer. The density of the marks is defined by an interspacing present between the tracks and an interspacing present between the marks on a track. Besides the standard CD, which can contain an audio program of well over an hour, a high-density CD (HDCD) is known having an information layer where the interspacings present between the tracks, the interspacings present between the marks on a track, and the pits themselves are substantially smaller than on a standard CD. The amount of information which the high-density CD can accommodate is thus substantially greater than the amount of information on the standard CD.
Since the available space on a high-density CD is comparatively large, the information layer of a high-density CD can be provided with more than one portion, such as, for example, a first portion and a second portion, the information of the first portion being suitable, for example, for being read out at a first read-out speed and the information of the second portion being suitable, for example, for being read out at a second read-out speed which is substantially higher than the first read-out speed. The term "read-out speed" is understood to mean the number of elementary marks to be read out per unit time, also called bit rate in the case of a CD. The read-out speed of each of the two portions is determined inter alia by the nature of the information present on each portion of the information layer.
A disadvantage of a disc-shaped information carrier of the kind mentioned in the opening paragraph is that the information carrier can only be fully read out by means of an optical read-out unit which has a comparatively wide range of rotational speeds.